FET Amplifier with protective circuit

ABSTRACT

The present invention relates to an acoustic transducer including an improved electret assembly mounted in an improved case and having an electronic amplifier circuit comprising FET transistors. The electronic circuit further includes means for protecting the FET transistors against current surges.

This is a division of application Ser. No. 755,468, filed Dec. 30, 1976,now U.S. Pat. No. 4,063,505.

BACKGROUND OF THE INVENTION

The present invention relates to an acoustic transducer of the typecomprising an electret assembly including a diaphragm mounted adjacentan electret film and a backplate. The electret assembly is mountedwithin a case to form acoustic chambers on opposite sides of thediaphragm. The case includes a conduit for permitting the externalacoustic signal to enter into one of the acoustic chambers to enable thediaphragm to respond thereto. The backplate is perforated to permit theair pulsations created by the vibrations of the diaphragm to pass to theother acoustic chamber. And, the electret film is connected to suitableelectronic circuitry to thereby permit the electroacoustical interactionof the diaphragm and electret film to provide an electrical signalrepresentative of the acoustic signal.

As is known, the converse operation may be provided by the transducer inthat an electrical signal may be applied to the electret to cause thediaphragm to vibrate and thereby develop an acoustic signal which can becoupled out of the first acoustic chamber.

A principal object of the present invention is to provide an improvedconstruction for an acoustic transducer of the foregoing type. In theinventive transducer, the diaphragm, electret and backplate assembly areconveniently mounted on support posts formed in the case; this alsoforms an acoustic chamber into which external sound is admitted. Thediaphragm includes flanges extending transversely from the plane of thediaphragm to permit secure adhesion thereof to the walls of the case.

The electret comprises a dielectric film deposited on a perforatedbackplate and the backplate includes protrusions which selectively spacethe dielectric film from a diaphragm. The foregoing structure permitsthe diaphragm, electret and backplate assembly to be mounted in the casein an acoustically efficient manner within a precise tolerance.

An electronic signal amplification circuit formed on a semiconductor dieis mounted on a ceramic plate which is positioned within the case and isin part supported by a post attached to the backplate. The electroniccircuit includes a pair of FET's (field effect transistors) and meansfor limiting current surges through the FET's. An electrical terminalcomprising relatively rigid electrical leads connecting to theelectronic circuit extend outwardly from the ceramic plate and throughacoustically sealed apertures in the case to be connected to theassociated external electronic circuitry.

The foregoing and other features, objects and advantages of the presentinvention will be apparent from the following more particulardescription of the preferred embodiment of the invention as illustratedin the accompanying drawings wherein:

DESCRIPTION OF THE DRAWINGS

FIG. 1 is an isometric view, partially in cross section, of an electrettransducer in accordance with the invention;

FIG. 2 is a plan view of the case of FIG. 1 with a portion of theelectret assembly and diaphragm removed to show a support post in thecasing, on which post the electret assembly is mounted. FIG. 2 alsoshows the ceramic plate on which the electronic circuit is mounted andthe base support for the ceramic plate;

FIG. 3 is a schematic diagram of a portion of the amplification circuitof FIG. 2 which includes a pair of field effect transistors (FET's) andassociated circuit components; and,

FIG. 4 is an isometric view of the ceramic plate 41 of FIGS. 1 and 2,essentially upside down or reversed with respect to FIGS. 1 and 2, toshow the component mounting on plate 41.

DESCRIPTION OF THE INVENTION

Referring to FIG. 1, the electret transducer 11 of the inventioncomprises a cup-like case or housing 13, which in one embodimentincludes essentially rectangularly shaped walls 15. A mating cover 17which comprises a generally flat plate with downwardly angled or slopedcorners 18 fits atop the walls 15 and is cemented thereon to close thecase 13. The sloped corners 18 assist in conveniently and properlylocating the cover 17 on the case 13 during the assembly operation. Asuitable acoustical signal input tube 19 mounted on case 13 communicatesthrough a suitable acoustical opening, indicated generally as numeral 40in end wall 15 with the interior of the case, and more particularly withacoustic chamber 20 formed in case 13, as will be explained.

An electret transducer assembly 21 is mounted in case 13. The electretassembly is generally of the type described in U.S. Pat. No. 3,740,496entitled "Diaphragm Assembly For Electret Transducer," filed in thenames of E. V. Carlson and M. C. Killion and assigned to the sameassignee as the present invention. Electret assembly 21 includes a onepiece diaphragm 22 formed as a plate-like portion 23 which extendsacross the entire relatively flat bottom or lower surface 32 of case 13,and a surround 25. Diaphragm 22 may be of polyethylene terephthalate,commonly available under the trademark MYLAR or of any similar material.Plate-like portion 23 may be coated with a metallizing layer ofconductive material, not shown, which may for example, be evaporated toits surface.

The surround 25 formed on the periphery of the plate-like portion 23,may also be metallized to provide an acoustical terminal. The surround25 includes a peripheral flange 26, extending transversely to the planeof plate-like portion 23. The flange 26 is formed to conform to theinner surface of the walls 15 of case 13. An integral tab which may beformed as an extension of flange 26 extends upward between the wall 15and the electrical conductor connected to terminal 51, note conductivetab 73 in FIGS. 2 and 3. The upper surface of diaphragm 22 is therebyelectrically connected to terminal 51 and the case 13 by the weld at 75.

A relatively rigid backplate 33 also extends across substantially theentire bottom or lower surface 32 of case 13. Backplate 33 is alsodimensioned to fit or conform to the flange 26 on diaphragm 22. As seenfrom FIG. 1, the flange 26 of diaphragm 22 is disposed and adhered inposition between the edges of backplate 33 and walls 15 of case 13. Thediaphragm 22 and backplate 33 divide the case 13 into two acousticchambers 20 and 24. Suitable apertures 35 in backplate 33 permit soundvibrations to pass from one acoustic chamber to the other.

A polarized dielectric film or electret 31 is formed on the lowersurface of backplate 33 by suitable known means. In one embodiment, theelectret film 31 may be placed or deposited on backplate 33 such as toflow through apertures 35 and extend to the opposite surface ofbackplate 33 to provide anchoring tabs for the film 31, for example, asdescribed in U.S. Pat. No. 3,772,133, filed in the name of Thomas H.Schmitt, entitled "Backplate Construction For Electret Transducer," andassigned to the same assignee as the present invention. A flange 30 onfilm 31 may extend around the edge of backplate 33. Backplate 33 furtherincludes a pattern of spaced protrusions 37 on its lower surface.Diaphragm plate-like portion 23 bears against protrusions 37 to properlyposition; that is to provide a precise spacing 42, between the diaphragmplate-like portion 23 and the section of the electret film 31 locatedbetween the protrusions.

A plurality of supporting posts 16, see also FIG. 2, are formed ormounted in a spaced pattern throughout the bottom or interior lowersurface 32 of case 13. In the embodiment of FIG. 1, three posts 16 intriangular configuration, that align with three protrusions of the type37 on the backplate 33 are utilized to support and securely mount theelectret assembly 21 within the case 13. As mentioned, the posts 16 maybe formed as indentations on the bottom 32 by depressing areas of thebottom 32 toward the interior of case 13. Alternatively, the posts 16may be separate posts or support stubs welded or otherwise secured inselected pattern on the bottom 32 of case 13. Note that in addition toproviding a secure support for assembly 21, the posts accurately definethe dimensions of the acoustic chamber 20 which are formed between thediaphragm 22 and the bottom of case 13.

In assembly, the diaphragm 22 is positioned adjacent to the backplate33. As explained above, the spacing between the diaphragm plate-likeportion 23 and the planar portions of backplate 33 will be controlled bythe protrusions 37 on backplate 33. The electret assembly or subassembly21 comprising the backplate 33 and diaphragm 22 can then be insertedinto the case 13 to rest on the posts 16 on the interior surface of thecase to form the acoustic chamber 20, as noted above. The backplate 33and the flange 26 of diaphragm 22 can then be secured to the walls 15 ofcase 13 such as by a nodule or fillet of glue 36.

A support plate 41 which may be of ceramic or other non-conductivematerial is mounted within the case 13 in spaced relation to thebackplate 33, see both FIGS. 1 and 2. The plate 41 is positioned andmounted by an elongated rod 43 extending outwardly from one edge of theplane of plate 41. Rod 43 is fastened as by welding onto a transversebrace 45, which may be struck out from backplate 33. Rod 43 may besupported on the side of brace 45, as shown, or also may conveniently besupported on top of the brace 45. Also, brace 45 can comprise a separatepiece welded onto both backplate 33 and rod 43.

The other edge of ceramic plate 41 is mounted in the case 13 by means ofrelatively rigid electrical terminals 47, 49 and 51 each of which have aportion affixed to plate 41 and an opposite portion which extends as bycementing to terminal pads 54 on the insulating board 52 mounted to wall15 of case 13. In addition, the plate 41 may be further secured in placeby adhesive 77 placed between the sides of the plate and the wall of thecase 13. The ends of the electrical terminals 47, 49 and 51 which areaffixed to plate 41, also connect to the associated electronic circuitry50, indicated in FIG. 3, which is mounted on plate 41. As mentionedabove, numeral 73 in FIG. 2 indicates a grounding tab formed on theflange 26 of the diaphragm 22 for electrically connecting with terminal51; and, reference numeral 75 indicates a weld point from terminal 51 tothe case 13.

The electronic circuitry of FIG. 3 includes a pair of field effecttransistors labeled FET-1 and FET-2, a pair of diodes 53 and 55 and aresistor 57. The circuit of FIG. 3 is a modification of the circuitdescribed in U.S. Pat. No. 3,512,100 filed in the names of M. C.Killion, E. V. Carlson and M. D. Burkhard entitled "Audio FrequencyAmplification Circuit" and assigned to the same assignee as the presentinvention.

In the embodiment shown in FIG. 3, FET-1 includes a drain electrode 65,a source electrode 67 and a gate electrode 63. The drain electrode 65 isconnected through terminal 47 to a suitable power source, such as abattery or cell, not shown, having a voltage in the range of 0.9 to 20.0volts and commonly between 0.9 and 2 volts.

The source electrode 67 is connected to the drain electrode 71 of FET-2.As is well known, FET-1 receives and amplifies the signal developed bythe interaction of the diaphragm 22 and the electret 31.

The source electrode 74 of FET-1 is connected through a series resistor57 to the other terminal 51 of the power source; terminal 51 is alsoconnected through lead 61 and conductor 73 (shown in dotted lines), tothe signal input source 60. Pad 58 shown in FIG. 4 indicates a conductorconnecting to the amplifier circuit 50. Resistor 57 is selected toprovide a voltage drop of between 10 millivolts and 100 millivolts whennormal current is flowing therethrough. For instance, in an amplifiercircuit which draws a 25 micro-amperes of current from the power supplya 1000 ohm resistor has been found to be effective in preventing damageto the circuit due to accidental electrostatic discharge through theterminals 47 and 49 when holding onto the case 13. The resistor 57 maybe formed as part of a semiconductor die 48, note FIG. 4; or,alternatively resistor 57 may be a discrete component, or also theresistor 57 may also be deposited on plate 41 by printing orevaporation.

Gate electrodes 63 of FET-1 is connected through parallel connecteddiodes 53 and 55 to the source electrode 74 of FET-2. The diodes 53 and55 are connected in relatively opposite polarity and function in thesame manner as disclosed in the above-cited U.S. Pat. No. 3,512,100.Gate electrode 69 of FET-2 also connects to diodes 53 and 55.

FIG. 4 indicates the amplifier circuit of FIG. 3 as comprising a singlesemiconductor die 48, which is mounted as by cementing or eutecticbonding to suitable printed circuitry on ceramic plate 41.

The circuit of FIG. 3 is an improvement of the circuit shown in U.S.Pat. No. 3,512,100 in that the second transistor FET-2 which functionsas a load transistor is connected in the circuit. The signal output isobtained from terminals 49 and 51 across FET-2 which provides a low DCresistance and a high AC resistance. In addition, the improved circuitof FIG. 3 includes the series resistor 57 which is connected in thecircuit to limit the maximum current flow through FET-1 or FET-2 in theevent of electrostatic discharge to either the power supply terminal 47or to the output terminal 49.

The input signal to the circuit 50 as indicated by a block 60 connectedacross the leads 28 and 73. As is known, the signal input to the circuitis dependent on the dynamic or instantaneous spacing of the diaphragmportion 23 with respect to the electret 31. The output of circuit 50 isan amplified electrical signal developed across terminals 49 and 51which are representative of the input signal.

As is well known, the polarity of the terminals of the power sourceconnected to each of the FET's depends on the polarity orientation ortype of FET's used.

While the invention has been particularly shown and described withreference to a preferred embodiment thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the spirit and scope of theinvention.

We claim:
 1. An amplification circuit connectable to a voltage powersource in the range of 0.9 to 2.0 volts, comprising at least two fieldeffect transistors (FET's) each having at least three electrodes, oneelectrode of the first FET connected to one terminal of the powersource, a diode biasing means comprising at least two diodes connectedin parallel, said diodes being respectively connected with a polarity topass current in relatively opposite directions, the second electrode ofsaid first FET connected for receiving an AC signal and also connectedto one terminal of the biasing means, the third electrode of said firstFET connected to the first electrode of said second FET, the second andthird electrodes of said second FET connected to the other terminal ofthe biasing means, means for coupling out the amplified AC signalgenerated across the first to the third electrodes of said second FETwhereby said second FET functions to provide a low DC load impedance anda high AC load impedance, and a resistance means connected from thesecond and third electrodes of said second FET to the other terminal ofthe power source, whereby said resistance means is effectively connectedin series with the DC current path traceable from the first to the thirdelectrodes of each said FET's and between the diode biasing means andthe other terminal of the power source to thereby limit the current flowthrough said FET's and also through said diode biasing means to provideprotection therefor such as against electrostatic discharges.